In a typical alarm system within a building, such as a fire or burglar alarm system, many types of sensors, detectors, lights, strobes, sounders and other associated devices may be located throughout the building as part of the system. Groups of these devices are often wired together along a pair of bidirectional communication lines. A group of such devices on a pair of lines is often referred to as a "line of devices". Many lines of devices can connect back to a control panel that controls the overall operation of the alarm system. A line of devices is usually associated with a certain zone of the building and/or a certain type of device. For example, one floor of a multi-story building may have all of its smoke detectors wired together on a line that connects back to the control panel.
Each device on a line may be individually addressed from the control panel. Individual addressing of devices allows a single device to indicate an alarm condition at a specific location on a line, and provides selective operation of specific devices, and can also be useful for alarm system fault diagnosis and/or individual device testing. Device addresses may be manually set in each device, for example, by configuring DIP switches or jumpers internal to the device to reflect a unique address code. However, during the installation of many devices, an installer may accidently configure the same address in more than one device. The problem of duplicate addressing may cause a failure in device communications resulting in incorrect operation of the alarm system.
Schemes have been developed to automatically and remotely assign addresses to devices in a predetermined order from the control panel to prevent duplicate address problems. For example, in Morita, U.S. Pat. No. 5,594,417, an alarm system is disclosed that includes an address setting unit in the control panel which sends special commands to devices in order to set device addresses. Each device initially contains a unique factory preset address. After installation, the address setting unit assigns unique address codes to each device and tracks allocated addresses. New devices may be added or devices may be removed and the address setting unit can use communications to electronically reset and reassign new addresses. Due to its automated nature, the address assignment scheme in Morita does prevent assignment of a new device address to more than one device. However, the solution is complicated since each device must contain a mechanism to set its own address. Moreover, Morita uses specialized devices, thus preventing alarm systems equipped with devices that use manual address setting mechanisms to benefit from the features of Morita.
During alarm system device installation, it is also important to properly connect and wire the devices to the lines. An improperly wired device may not have enough electrical contact with a line to allow for effective communications with the control panel. Devices that are poorly wired may become difficult to communicate with over time due to aging of the electrical connection, corrosion, dirt and/or dust build up which increases resistance in the line connection.
Various ways have been proposed to ensure that devices are properly connected to the lines. For instance, Weston et al., U.S. Pat. No. 4,777,473, discloses an addressable device alarm system in which each device contains an internal alarm condition resistor. When the control panel sends a message to a specific device address, the device having that address connects its internal alarm condition resistor to the line and the control panel measures the resistance on the line, including the alarm condition resistor in the device. If the resistance measured is as expected, the device is determined to be correctly wired to the line. However, as in Morita, devices in Weston require special circuitry to engage the internal resistor with the line when instructed by the control panel.